As a result of the energy crisis, there has been a considerable interest in developing alternative sources of energy for electric power stations to replace increasingly expensive imported petroleum products. One method which has generated considerable interest is the use of solar energy as a heat source for generating steam for electric power generation. In this process, it is necessary to concentrate the energy coming from the sun to heat a liquid which absorbs the heat and carries it to the generator. In relatively low power hot water or space heating systems, the liquid is water which is either pumped or fed by gravity through a plurality of solar panels to a storage medium, generally crushed rock, from which the heat is extracted for subsequent use. Such systems are beginning to find wide use in home heating and hot water applications, particularly in the Southwest where long periods of sunlight are the norm. However, when it is desired to use solar energy as a source for large scale electric power application, the relatively low boiling point of water makes its use unsatisfactory unless high pressure components are used throughout the system. It would be much better if a low pressure system could be used.
One type of thermal transfer medium which has evoked considerable interest is mixed salts having a relatively large spread between their melting and boiling points. Many of these have relatively high specific heats so that they are able to absorb large quantities of thermal energy per degree of temperature rise more efficiently than many types of metallic or organic heat transfer media. When liquified, these salts can be readily pumped to and through a "power tower" or similar heat concentration system to obtain a large quantity of sensible heat which is then utilized in a more-or-less conventional steam generator/turbine system for electric power generation. For such use, a number of salt mixtures are known which are stable at temperatures of up to about 900.degree. F.
One particular salt mixture which has been proposed as a practical thermal storage medium for these purposes is a eutectic mixture of sodium nitrate and potassium nitrate. Such a mixture meets the above-stated criteria in that its melting point is relatively low, being about 400.degree. F. and, when molten, is both quite fluid and, most critically, stable at temperatures of up to about 1200.degree. F. Furthermore, the eutectic composition, unlike those of most metallic alloys, is quite broad, ranging from about 30% to about 70% by weight of sodium nitrate in the mix. Thus, tolerance to minor changes in the sodium/potassium ratio is extremely high. Further, the raw materials are widely available and relatively low in cost.
However, to avoid potential operating problems with stress corrosion and line pluggage, it is necessary that both components of the mix be essentially free of chloride and high melting point carbonate, oxide and sulfate impurities, with a combined maximum total purity level of about 0.7% being considered acceptable and with the chloride level being no more than about 0.2%. Further, a number of problems have arisen in producing such a mixed salt product in a form which allows the safe and easy shipment of the large quantities required to the relatively remote areas, such as those in the Southwest, wherein many large solar power stations have been proposed. It has been found that unless the mixed salt is produced in a relatively non-friable form, essentially free of fines (less than about 50 mesh), it is quite sensitive to moisture level in excess of about 0.5%. Levels higher than this tend to cause the salt to set up and cake quite badly. In such condition, it cannot be shipped in large quantities, specifically in covered railroad hopper cars and barges, without encountering significant problems, first in unloading the material and then in crushing it, after long-term storage, prior to charging it into the power system. This is not an unusual problem and is frequently encountered in the shipment of many bulk chemicals. However, solutions developed such as bagging or drumming almost invariably add both to production and shipping costs.
What is needed is a method for producing a dry homogeneous eutectic blend of sodium nitrate and potassium nitrate which is non-friable, free-flowing and relatively insensitive to moisture when shipped and stored in bulk quantities.